1. Field of the Invention
The present invention relates to a bulk superconductor comprising a superconductive REBa.sub.2 Cu.sub.3 O.sub.7-x phase.
2. Description of the Related Art
At the present, enlargement of a crystal of REBa.sub.2 Cu.sub.3 O.sub.7-x (123 phase type) superconductor is sought after. Here, RE represents Y or rare earth element. Such enlargement of a crystal should be made in a single crystal of the (123 ) phase since a polycrystalline structure involves crystal grain boundaries between (123) phases that weaken the bonds between the crystals thereby deteriorating the superconductor properties.
It has been proposed to prepare a stack of layers comprising different complex oxides of RE, Ba and Cu that are arranged such that (123) phase formation temperatures thereof are successively lowered using the repetitive quench method or by the repetitive pressing of starting oxide powder mixtures, followed by heating the stack to a partial melt state, seeding the stack with a seed crystal and then unidirectionally crystallizing the stack of layers utilizing the difference in the (123) phase formation temperature (Advances in Superconductivity III, Springier-Verlag. Tokyo, 1990, pp 733-736).
In the above proposal, the arrangement of layers is made to provide a uniaxial gradient of (123) phase formation temperature (Tf) by the composition difference, particularly RE, of the starting materials. In this case, a seed crystal is disposed at the center of a top layer of the stack and, for example, the edges or periphery of the top layer, remote from the seed crystal, tend to be nucleated, which results in the formation of a polycrystal of the (1,2,3) phase.
The object of the present invention is to solve the above problem and to provide a bulk REBa.sub.2 Cu.sub.3 O.sub.7-x superconductor in the form of a single crystal of the between (123) phase wherein precipitates of a (211) phase are dispersed.